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<tr class="t-nv-h1"><td colspan="5"> Utilities</td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
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<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
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</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="declarations.html" title="cpp/language/declarations"> Declarations</a><div class="t-navbar-menu"><div><div style="display:inline-block">
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<tr class="t-nv-h2"><td colspan="5"> Overview </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> declaration syntax</a> </td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="typedef.html" title="cpp/language/typedef"><span class="t-lines"><span>typedef</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="consteval.html" title="cpp/language/consteval"><span class="t-lines"><span>consteval</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="alignas.html" title="cpp/language/alignas"><span class="t-lines"><span>alignas</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="cv.html" title="cpp/language/cv"><span class="t-lines"><span>const</span><span>volatile</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="elaborated_type_specifier.html" title="cpp/language/elaborated type specifier"> elaborated type specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="attributes.html" title="cpp/language/attributes"> attributes </a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Declarators </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="reference.html" title="cpp/language/reference"> reference </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="pointer.html" title="cpp/language/pointer"> pointer </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="array.html" title="cpp/language/array"> array </a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Block declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> simple-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> →<strong class="selflink"> structured binding declaration</strong> <span class="t-mark-rev t-since-cxx17">(C++17)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type_alias.html" title="cpp/language/type alias"> <code>alias</code> declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace_alias.html" title="cpp/language/namespace alias"> namespace alias definition </a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="namespace.html#Using-directives" title="cpp/language/namespace"> using-directive</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_assert.html" title="cpp/language/static assert"> <code>static_assert</code> declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> asm-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="enum_class.html" title="cpp/language/enum"> opaque enum declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Other declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace.html" title="cpp/language/namespace"> namespace definition</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html" title="cpp/language/function"> function declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> class template declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> function template declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html#Explicit_instantiation" title="cpp/language/class template"> explicit template instantiation</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> explicit template specialization</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="language_linkage.html" title="cpp/language/language linkage"> linkage specification</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> attribute declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> empty declaration</a> </td></tr>
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</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Binds the specified names to subobjects or elements of the initializer.
</p><p>Like a reference, a structured binding is an alias to an existing object. Unlike a reference, a structured binding does not have to be of a reference type.
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">cv-auto ref-qualifier</span><span class="t-mark">(optional)</span> <code><b>[</b></code> <span class="t-spar">identifier-list</span> <code><b>] = </b></code><span class="t-spar">expression</span> <code><b>;</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">cv-auto ref-qualifier</span><span class="t-mark">(optional)</span> <code><b>[</b></code> <span class="t-spar">identifier-list</span> <code><b>]{</b></code> <span class="t-spar">expression</span> <code><b>};</b></code>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">cv-auto ref-qualifier</span><span class="t-mark">(optional)</span> <code><b>[</b></code> <span class="t-spar">identifier-list</span> <code><b>](</b></code> <span class="t-spar">expression</span> <code><b>);</b></code>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">attr</span>
</td>
<td> -
</td>
<td> sequence of any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">cv-auto</span>
</td>
<td> -
</td>
<td> possibly cv-qualified type specifier <code><b>auto</b></code><span class="t-rev-inl t-since-cxx20"><span>, may also include <a href="storage_duration.html" title="cpp/language/storage duration">storage-class-specifier</a> <code><b>static</b></code> or <code><b>thread_local</b></code>; including <code><b>volatile</b></code> in cv-qualifiers is deprecated</span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">ref-qualifier</span>
</td>
<td> -
</td>
<td> either <code><b>&amp;</b></code> or <code><b>&amp;&amp;</b></code>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">identifier-list</span>
</td>
<td> -
</td>
<td> list of comma-separated identifiers introduced by this declaration
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">expression</span>
</td>
<td> -
</td>
<td>  an expression that does not have the comma operator at the top level (grammatically, an <i>assignment-expression</i>), and has either array or non-union class type. If <span class="t-spar">expression</span> refers to any of the names from <span class="t-spar">identifier-list</span>, the declaration is ill-formed.
</td></tr></table>
<p>A structured binding declaration introduces all identifiers in the <span class="t-spar">identifier-list</span> as names in the surrounding scope and binds them to subobjects or elements of the object denoted by <span class="t-spar">expression</span>. The bindings so introduced are called <i>structured bindings</i>.
</p><p>A structured binding declaration first introduces a uniquely-named variable (here denoted by <code><b><i>e</i></b></code>) to hold the value of the initializer, as follows:
</p>
<ul><li> If <span class="t-spar">expression</span> has array type <code>A</code> and no <span class="t-spar">ref-qualifier</span> is present, then <code><b><i>e</i></b></code> has type <i>cv</i> <code>A</code>, where <i>cv</i> is the cv-qualifiers in the <span class="t-spar">cv-auto</span> sequence, and each element of <code><b><i>e</i></b></code> is copy- (for <span class="t-v">(1)</span>) or direct- (for <span class="t-v">(2,3)</span>) initialized from the corresponding element of <span class="t-spar">expression</span>.
</li><li> Otherwise <code><b><i>e</i></b></code> is defined as if by using its name instead of <code><b>[</b></code> <span class="t-spar">identifier-list</span> <code><b>]</b></code> in the declaration.
</li></ul>
<p>We use <code><b><i>E</i></b></code> to denote the type of the expression <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span>. (In other words, <code><b><i>E</i></b></code> is the equivalent of <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/remove_reference.html"><span class="kw609">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>decltype<span class="br0">(</span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span><span class="sy1">&gt;</span></span></span>.) 
</p><p>A structured binding declaration then performs the binding in one of three possible ways, depending on <code><b><i>E</i></b></code>:
</p>
<ul><li> Case 1: if <code><b><i>E</i></b></code> is an array type, then the names are bound to the array elements.
</li><li> Case 2: if <code><b><i>E</i></b></code> is a non-union class type and <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/tuple/tuple_size.html"><span class="kw1104">std::<span class="me2">tuple_size</span></span></a><span class="sy1">&lt;</span>E<span class="sy1">&gt;</span></span></span> is a complete type with a member named <code>value</code> (regardless of the type or accessibility of such member), then the "tuple-like" binding protocol is used.
</li><li> Case 3: if <code><b><i>E</i></b></code> is a non-union class type but <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/tuple/tuple_size.html"><span class="kw1104">std::<span class="me2">tuple_size</span></span></a><span class="sy1">&lt;</span>E<span class="sy1">&gt;</span></span></span> is not a complete type, then the names are bound to the accessible data members of <code><b><i>E</i></b></code>.
</li></ul>
<p>Each of the three cases is described in more detail below. 
</p><p>Each structured binding has a <i>referenced type</i>, defined in the description below. This type is the type returned by <a href="decltype.html" title="cpp/language/decltype"><code>decltype</code></a> when applied to an unparenthesized structured binding.
</p>
<h4><span class="mw-headline" id="Case_1:_binding_an_array">Case 1: binding an array</span></h4>
<p>Each identifier in the <span class="t-spar">identifier-list</span> becomes the name of an lvalue that refers to the corresponding element of the array. The number of identifiers must equal the number of array elements.
</p><p>The <i>referenced type</i> for each identifier is the array element type. Note that if the array type <code><b><i>E</i></b></code> is cv-qualified, so is its element type.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">auto</span> <span class="br0">[</span>x, y<span class="br0">]</span> <span class="sy1">=</span> a<span class="sy4">;</span>    <span class="co1">// creates e[2], copies a into e,</span>
                    <span class="co1">// then x refers to e[0], y refers to e[1]</span>
<span class="kw4">auto</span><span class="sy3">&amp;</span> <span class="br0">[</span>xr, yr<span class="br0">]</span> <span class="sy1">=</span> a<span class="sy4">;</span> <span class="co1">// xr refers to a[0], yr refers to a[1]</span></pre></div></div>
<h4><span class="mw-headline" id="Case_2:_binding_a_tuple-like_type">Case 2: binding a tuple-like type</span></h4>
<p>The expression <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/tuple/tuple_size.html"><span class="kw1104">std::<span class="me2">tuple_size</span></span></a><span class="sy1">&lt;</span>E<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span></span></span> must be a well-formed integer constant expression, and the number of identifiers must equal <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/tuple/tuple_size.html"><span class="kw1104">std::<span class="me2">tuple_size</span></span></a><span class="sy1">&lt;</span>E<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span></span></span>.
</p><p>For each identifier, a variable whose type is "reference to <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/tuple/tuple_element.html"><span class="kw1102">std::<span class="me2">tuple_element</span></span></a><span class="sy1">&lt;</span>i, E<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span></span></span>" is introduced: lvalue reference if its corresponding initializer is an lvalue, rvalue reference otherwise. The initializer for the i-th variable is
</p>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">e.<span class="me1">get</span><span class="sy1">&lt;</span>i<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span></span></span>, if lookup for the identifier <code>get</code> in the scope of <code><b><i>E</i></b></code> by class member access lookup finds at least one declaration that is a function template whose first template parameter is a non-type parameter
</li><li> Otherwise, <span class="t-c"><span class="mw-geshi cpp source-cpp">get<span class="sy1">&lt;</span>i<span class="sy1">&gt;</span><span class="br0">(</span>e<span class="br0">)</span></span></span>, where <span class="t-c"><span class="mw-geshi cpp source-cpp">get</span></span> is looked up by <a href="adl.html" title="cpp/language/adl">argument-dependent lookup</a> only, ignoring non-ADL lookup.
</li></ul>
<p>In these initializer expressions, <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> is an lvalue if the type of the entity <code><b><i>e</i></b></code> is an lvalue reference (this only happens if the <span class="t-spar">ref-qualifier</span> is <code><b>&amp;</b></code> or if it is <code><b>&amp;&amp;</b></code> and the initializer expression is an lvalue) and an xvalue otherwise (this effectively performs a kind of perfect forwarding), <span class="t-c"><span class="mw-geshi cpp source-cpp">i</span></span> is a <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/size_t.html"><span class="kw106">std::<span class="me2">size_t</span></span></a></span></span> prvalue, and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;</span>i<span class="sy1">&gt;</span></span></span> is always interpreted as a template parameter list. 
</p><p>The variable has the same <a href="storage_duration.html" title="cpp/language/storage duration">storage duration</a> as <code><b><i>e</i></b></code>.
</p><p>The identifier then becomes the name of an lvalue that refers to the object bound to said variable.
</p><p>The <i>referenced type</i> for the i-th identifier is <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/tuple/tuple_element.html"><span class="kw1102">std::<span class="me2">tuple_element</span></span></a><span class="sy1">&lt;</span>i, E<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span></span></span>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">float</span> x<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">char</span>  y<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span>   z<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<a href="../utility/tuple.html"><span class="kw1101">std::<span class="me2">tuple</span></span></a><span class="sy1">&lt;</span><span class="kw4">float</span><span class="sy3">&amp;</span>, <span class="kw4">char</span><span class="sy3">&amp;&amp;</span>, <span class="kw4">int</span><span class="sy1">&gt;</span> tpl<span class="br0">(</span>x, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>y<span class="br0">)</span>, z<span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> <span class="br0">[</span>a, b, c<span class="br0">]</span> <span class="sy1">=</span> tpl<span class="sy4">;</span>
<span class="co1">// using Tpl = const std::tuple&lt;float&amp;, char&amp;&amp;, int&gt;;</span>
<span class="co1">// a names a structured binding that refers to x (initialized from get&lt;0&gt;(tpl))</span>
<span class="co1">// decltype(a) is std::tuple_element&lt;0, Tpl&gt;::type, i.e. float&amp;</span>
<span class="co1">// b names a structured binding that refers to y (initialized from get&lt;1&gt;(tpl))</span>
<span class="co1">// decltype(b) is std::tuple_element&lt;1, Tpl&gt;::type, i.e. char&amp;&amp;</span>
<span class="co1">// c names a structured binding that refers to the third component of tpl, get&lt;2&gt;(tpl)</span>
<span class="co1">// decltype(c) is std::tuple_element&lt;2, Tpl&gt;::type, i.e. const int</span></pre></div></div>
<h4><span class="mw-headline" id="Case_3:_binding_to_data_members">Case 3: binding to data members</span></h4>
<p>Every non-static data member of <code><b><i>E</i></b></code> must be a direct member of <code><b><i>E</i></b></code> or the same base class of <code><b><i>E</i></b></code>, and must be well-formed in the context of the structured binding when named as <code><i><b>e</b></i>.<i>name</i></code>. <code><b><i>E</i></b></code> may not have an anonymous union member. The number of identifiers must equal the number of non-static data members.
</p><p>Each identifier in <span class="t-spar">identifier-list</span> becomes the name of an lvalue that refers to the next member of <code><b><i>e</i></b></code> in declaration order (bit fields are supported); the type of the lvalue is that of <span class="t-c"><span class="mw-geshi cpp source-cpp">e.<span class="me1">m_i</span></span></span>, where <code>m_i</code> refers to the i-th member.
</p><p>The <i>referenced type</i> of the i-th identifier is the type of <span class="t-c"><span class="mw-geshi cpp source-cpp">e.<span class="me1">m_i</span></span></span> if it is not a reference type, or the declared type of <code>m_i</code> otherwise.
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw1">struct</span> S
<span class="br0">{</span>
    mutable <span class="kw4">int</span> x1 <span class="sy4">:</span> <span class="nu0">2</span><span class="sy4">;</span>
    <span class="kw4">volatile</span> <span class="kw4">double</span> y1<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
S f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> S<span class="br0">{</span><span class="nu0">1</span>, <span class="nu16">2.3</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>x, y<span class="br0">]</span> <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// x is an int lvalue identifying the 2-bit bit field</span>
                             <span class="co1">// y is a const volatile double lvalue</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> y <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>  <span class="co1">// 1 2.3</span>
    x <span class="sy1">=</span> <span class="sy2">-</span><span class="nu0">2</span><span class="sy4">;</span>   <span class="co1">// OK</span>
<span class="co1">//  y = -2.;  // Error: y is const-qualified</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> y <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>  <span class="co1">// -2 2.3</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1 2.3
-2 2.3</pre></div></div> 
</div>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>The lookup for member <code>get</code> ignores accessibility as usual and also ignores the exact type of the non-type template parameter. A private <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy2">*</span><span class="sy1">&gt;</span> <span class="kw4">void</span> get<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> member will cause the member interpretation to be used, even though it is ill-formed.
</p><p>The portion of the declaration preceding <code><b>[</b></code> applies to the hidden variable <code><b><i>e</i></b></code>, not to the introduced identifiers.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span>, b <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> <span class="br0">[</span>x, y<span class="br0">]</span> <span class="sy1">=</span> <a href="../utility/tuple/tie.html"><span class="kw1116">std::<span class="me2">tie</span></span></a><span class="br0">(</span>a, b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// x and y are of type int&amp;</span>
<span class="kw4">auto</span> <span class="br0">[</span>z, w<span class="br0">]</span> <span class="sy1">=</span> <a href="../utility/tuple/tie.html"><span class="kw1116">std::<span class="me2">tie</span></span></a><span class="br0">(</span>a, b<span class="br0">)</span><span class="sy4">;</span>        <span class="co1">// z and w are still of type int&amp;</span>
<a href="../error/assert.html"><span class="kw775">assert</span></a><span class="br0">(</span><span class="sy3">&amp;</span>z <span class="sy1">==</span> <span class="sy3">&amp;</span>a<span class="br0">)</span><span class="sy4">;</span>                    <span class="co1">// passes</span></pre></div></div>
<p>The tuple-like interpretation is always used if <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/tuple/tuple_size.html"><span class="kw1104">std::<span class="me2">tuple_size</span></span></a><span class="sy1">&lt;</span>E<span class="sy1">&gt;</span></span></span> is a complete type, even if that would cause the program to be ill-formed:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw4">int</span> x<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">namespace</span> std
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
    <span class="kw1">struct</span> tuple_size<span class="sy1">&lt;</span><span class="sy4">::</span><span class="me2">A</span><span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">auto</span> <span class="br0">[</span>x<span class="br0">]</span> <span class="sy1">=</span> A<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// error; the "data member" interpretation is not considered.</span></pre></div></div>
<p>The usual rules for reference-binding to temporaries (including lifetime-extension) apply if a <span class="t-spar">ref-qualifier</span> is present and the <span class="t-spar">expression</span> is a prvalue. In those cases the hidden variable <code><b><i>e</i></b></code> is a reference that binds to the temporary variable <a href="implicit_cast.html#Temporary_materialization" title="cpp/language/implicit conversion">materialized</a> from the prvalue expression, extending its lifetime. As usual, the binding will fail if <code><b><i>e</i></b></code> is a non-const lvalue reference:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
 
<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> <span class="br0">[</span>x<span class="br0">]</span> <span class="sy1">=</span> <a href="../utility/tuple/make_tuple.html"><span class="kw1115">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, not dangling</span>
<span class="kw4">auto</span><span class="sy3">&amp;</span>       <span class="br0">[</span>y<span class="br0">]</span> <span class="sy1">=</span> <a href="../utility/tuple/make_tuple.html"><span class="kw1115">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error, cannot bind auto&amp; to rvalue std::tuple</span>
<span class="kw4">auto</span><span class="sy3">&amp;&amp;</span>      <span class="br0">[</span>z<span class="br0">]</span> <span class="sy1">=</span> <a href="../utility/tuple/make_tuple.html"><span class="kw1115">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// also OK</span></pre></div></div>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span>x<span class="br0">)</span></span></span>, where <code>x</code> denotes a structured binding, names the <i>referenced type</i> of that structured binding. In the tuple-like case, this is the type returned by <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/tuple/tuple_element.html"><span class="kw1102">std::<span class="me2">tuple_element</span></span></a></span></span>, which may not be a reference even though a hidden reference is always introduced in this case. This effectively emulates the behavior of binding to a struct whose non-static data members have the types returned by <code>tuple_element</code>, with the referenceness of the binding itself being a mere implementation detail.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><a href="../utility/tuple.html"><span class="kw1101">std::<span class="me2">tuple</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">auto</span> <span class="br0">[</span>x, y<span class="br0">]</span> <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// decltype(x) is int</span>
                         <span class="co1">// decltype(y) is int&amp;</span>
 
<span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>z, w<span class="br0">]</span> <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// decltype(z) is const int</span>
                         <span class="co1">// decltype(w) is int&amp;</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx20"><td>
<p>Structured bindings cannot be captured by <a href="lambda.html" title="cpp/language/lambda">lambda expressions</a>.
</p>
</td>
<td><span class="t-mark-rev t-until-cxx20">(until C++20)</span></td></tr>
</table>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;cassert&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">struct</span> S <span class="br0">{</span> <span class="kw4">int</span> p<span class="br0">{</span><span class="nu0">6</span><span class="br0">}</span>, q<span class="br0">{</span><span class="nu0">7</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
    <span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&amp;</span> <span class="br0">[</span>b, d<span class="br0">]</span> <span class="sy1">=</span> S<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
    <span class="kw4">auto</span> l <span class="sy1">=</span> <span class="br0">[</span>b, d<span class="br0">]</span> <span class="br0">{</span> <span class="kw1">return</span> b <span class="sy2">*</span> d<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// valid since C++20</span>
    <a href="../error/assert.html"><span class="kw775">assert</span></a><span class="br0">(</span>l<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
</div>
<h3><span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;set&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;iomanip&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../container/set.html"><span class="kw1280">std::<span class="me2">set</span></span></a><span class="sy1">&lt;</span><a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="sy1">&gt;</span> myset<span class="br0">{</span><span class="st0">"hello"</span><span class="br0">}</span><span class="sy4">;</span>
 
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i<span class="br0">{</span><span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> i<span class="sy4">;</span> <span class="sy2">--</span>i<span class="br0">)</span>
    <span class="br0">{</span>
        <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> <span class="br0">[</span>iter, success<span class="br0">]</span> <span class="sy1">=</span> myset.<span class="me1">insert</span><span class="br0">(</span><span class="st0">"Hello"</span><span class="br0">)</span><span class="sy4">;</span> success<span class="br0">)</span> 
            <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Insert is successful. The value is "</span>
                      <span class="sy1">&lt;&lt;</span> <a href="../io/manip/quoted.html"><span class="kw1806">std::<span class="me2">quoted</span></span></a><span class="br0">(</span><span class="sy2">*</span>iter<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">".<span class="es1">\n</span>"</span><span class="sy4">;</span>
        <span class="kw1">else</span>
            <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"The value "</span> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/quoted.html"><span class="kw1806">std::<span class="me2">quoted</span></span></a><span class="br0">(</span><span class="sy2">*</span>iter<span class="br0">)</span>
                      <span class="sy1">&lt;&lt;</span> <span class="st0">" already exists in the set.<span class="es1">\n</span>"</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="kw1">struct</span> BitFields
    <span class="br0">{</span>
        <span class="co1">// C++20: default member initializer for bit-fields</span>
        <span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">4</span> <span class="br0">{</span><span class="nu0">1</span><span class="br0">}</span>, d <span class="sy4">:</span> <span class="nu0">4</span> <span class="br0">{</span><span class="nu0">2</span><span class="br0">}</span>, p <span class="sy4">:</span> <span class="nu0">4</span> <span class="br0">{</span><span class="nu0">3</span><span class="br0">}</span>, q <span class="sy4">:</span> <span class="nu0">4</span> <span class="br0">{</span><span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
    <span class="br0">}</span><span class="sy4">;</span>
 
    <span class="br0">{</span>
        <span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>b, d, p, q<span class="br0">]</span> <span class="sy1">=</span> BitFields<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> b <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> d <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> p <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> q <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="br0">{</span>
        <span class="kw4">const</span> <span class="kw4">auto</span> <span class="br0">[</span>b, d, p, q<span class="br0">]</span> <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">{</span> <span class="kw1">return</span> BitFields<span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">3</span>, <span class="nu0">2</span>, <span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> b <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> d <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> p <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> q <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="br0">{</span>
        BitFields s<span class="sy4">;</span>
 
        <span class="kw4">auto</span><span class="sy3">&amp;</span> <span class="br0">[</span>b, d, p, q<span class="br0">]</span> <span class="sy1">=</span> s<span class="sy4">;</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> b <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> d <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> p <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> q <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
        b <span class="sy1">=</span> <span class="nu0">4</span>, d <span class="sy1">=</span> <span class="nu0">3</span>, p <span class="sy1">=</span> <span class="nu0">2</span>, q <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">b</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">d</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">p</span> <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">q</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Insert is successful. The value is "Hello".
The value "Hello" already exists in the set.
1 2 3 4
4 3 2 1
1 2 3 4
4 3 2 1</pre></div></div> 
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2285.html">CWG 2285</a>
</td>
<td> C++17
</td>
<td> <span class="t-spar">expression</span> could refer to the names from <span class="t-spar">identifier-list</span>
</td>
<td> the declaration is ill-formed in this case
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.link/P0961R1">P0961R1</a>
</td>
<td> C++17
</td>
<td> in the tuple-like case, member <code>get</code> is used<br>if lookup finds a <code>get</code> of whatever kind
</td>
<td> only if lookup finds a function template<br>with a non-type parameter
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.link/P0969R0">P0969R0</a>
</td>
<td> C++17
</td>
<td> in the binding-to-members case,<br>the members are required to be public
</td>
<td> only required to be accessible<br>in the context of the declaration
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2312.html">CWG 2312</a>
</td>
<td> C++17
</td>
<td> the meaning of <code>mutable</code> was lost<br>in the binding-to-members case
</td>
<td> its meaning is still kept
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2386.html">CWG 2386</a>
</td>
<td> C++17
</td>
<td> the "tuple-like" binding protocol is used<br>whenever <code>tuple_size&lt;E&gt;</code> is a complete type
</td>
<td> used only when <code>tuple_size&lt;E&gt;</code><br>has a member <code>value</code>
</td></tr></table>
<h3><span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-c++20">
<ul><li> C++20 standard (ISO/IEC 14882:2020): 
</li></ul>
<dl><dd><ul><li> 9.6 Structured binding declarations [dcl.struct.bind] (p: 219-220)
</li></ul>
</dd></dl>
<div class="t-ref-std-c++17">
<ul><li> C++17 standard (ISO/IEC 14882:2017): 
</li></ul>
<dl><dd><ul><li> 11.5 Structured binding declarations [dcl.struct.bind] (p: 219-220)
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../utility/tuple/tie.html" title="cpp/utility/tuple/tie"> <span class="t-lines"><span>tie</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td>   creates a <span class="t-lc"><a href="../utility/tuple.html" title="cpp/utility/tuple">tuple</a></span> of lvalue references or unpacks a tuple into individual objects  <br> <span class="t-mark">(function template)</span> </td></tr>
</table></div>

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